1. We've just seen a professor. He is going to give a lecture at our school tomorrow. 

->We've just seen a professor who is going to give a lecture at our school tomorrow

2. The man was painting the ceiling. He fell off the ladder.

->The man who was painting the ceiling fell off the ladder

3. The girl is very good at mathematics. She sits next to me. 

->The girl who is very good at mathematics sits next to me

4. I'm looking forward to meeting an old friend. I haven't seem him for two years.

->I'm looking forward to meeting an old friend who I haven't seem->seen for two years

5. We invited an actor to our school last Sunday. He gave a wonderful performance.

->We invited an actor who gave a wonderful performance to our school last Sunday

6. The police have caught the thieves. They stole my car five days ago.

->The police have caught the thieves who stole my car five days ago

7. The students have great difficulty in understanding each other. They don't have a common language

->The students who don't have a common language have great difficulty in understanding each other

8. The customer asked to see the manager. He complained about the service.

->The customer who complained about the service asked to see the manager

9. The man was  an experienced mechanic. I sent for him to repair my motorbike. 

->The man whom I sent to repair my motorbike was an experienced mechanic

10. The boy has just gone out. Do you know him?

->Do you know the boy who has just gone out?